1. Field of the Invention
This invention relates to an image processing apparatus for processing an image signal representing a diagnostic image in order to obtain a visible image suitable for diagnosis.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out 10 appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal- The image signal is then used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT. This system is especially preferable to obtain a diagnostic image where reduction of radiation dose is required.
In the aforesaid systems, various image processings are carried out on the image signal in order to obtain a visible image suitable for diagnostic purposes as disclosed, for instance in Japanese Unexamined Patent Publication No. 55(1980)-87983, U.S. Pat. No. 4,315,318, and U.S. patent application No. 259,814.
However when the image processing methods which have been proposed are applied to an actual image, it must be determined what spatial frequency is to be enhanced (or weakened) to what extent, and the like. In the systems which treats a large number of images, it is very troublesome and inefficient to determine the image processing condition for each image by trial and error. Accordingly, there has been commonly used a method in which the images are grouped and the image processing condition is determined in advance for each group. For example, in the systems for treating diagnostic images, the images are generally grouped by the recorded parts (e.g., head, neck, chest and abdomen) and/or the recording conditions (normal radiography, tomography and enlarged radiography).
For example, in the case of an image of the chest, the doctor sometimes wants to inspect the heart and sometimes wants to inspect the lung, and accordingly, the image processing condition has been determined so that a visible image having acceptable image quality over the entire chest can be reproduced. However this means that neither of the image of the heart nor the image of the lung has the best image quality.